1. Field of the Invention
This invention pertains to the field of fertilizer science and technology. The invention pertains, in particular, to the synthesis of ammonia by the catalytic reaction of hydrogen and nitrogen.
2. Description of the Prior Art
The synthesis of ammonia is a simple catalytic reaction in which three mols of hydrogen and one mol of nitrogen combine to form two mols of ammonia. The reaction is highly selective in that no byproducts are formed, the only concern being contaminants in the feed which might poison the catalyst, or inerts, such as methane and argon, which have to be purged from the system. Despite its simplicity, the ammonia synthesis reaction is intrinsically limited by thermodynamic equilibrium. The equilibrium studies first carried out by Fritz Haber in the early nineteen hundreds showed that the synthesis reaction required high pressures and low temperatures, but the extent of conversion was limited and considerable recycle of unreacted gas was required. The effects of the many process variables in the synthesis reaction have been studied by many investigators, as summarized for example in "An Investigation on Promoted Iron Catalysts for the Synthesis of Ammonia", Anders Nielsen, The Haldor Topsoe Research Laboratory, 3rd Edition.
The early synthetic ammonia plants were operated at pressures in excess of 500 atmospheres, but these plants were expensive to build and operate. In recent years ammonia plant installations have been in the 100 to 200 atmosphere pressure range. However, when operating at this lower pressure range at, for example, an 896.degree. F. temperature with 10% inerts in the feed, the equilibrium ammonia content is only 10 to 18% in the product compared to 34% at a 500 atmosphere operating pressure. Temperature also has a pronounced effect on the equilibrium ammonia content. For example, at 200 atmospheres pressure with 10% inerts in the feed, the equilibrium ammonia content at 694.degree. F. is 39% and at 925.degree. F. is only 16%. The effect of an increasing temperature to decrease the equilibrium ammonia content is aggravated in the ammonia process by the highly exothermic heat of the synthesis reaction. For example, when producing 10% ammonia in the product gas there is about a 260.degree. F. rise in temperature as the synthesis gas flows from the inlet to the outlet of the catalyst bed. In a typical catalyst bed in an ammonia reactor the inlet temperature is low and the equilibrium ammonia content is high. However, as a result of the exothermic heat of reaction the outlet temperature is high and the equilibrium ammonia content is low, with the result that the conversion of the reactants to ammonia is low. Furthermore, steps must be taken to decrease the gas temperature before feeding the next catalyst bed. This is typically done by heat exchange or quenching with cold synthesis gas. Typical synthesis reactors may have two to four catalyst beds, but the gas exiting the reactor contains only 10 to 15% ammonia.
It is apparent from the above that high recycle of unreacted feed is required in present day commercial designs. One undesirable result of low conversion to ammonia and high recycle is a build up of the inert content of the feed to the reactor. Inerts in the feed decrease the partial pressure of the reactants and, as a result, there is a decrease in the equilibrium ammonia content. Fresh synthesis gas may have only 1 to 1.5% inerts, as argon and unconverted methane, but will build up to 10 to 15% as a result of the low conversion and high recycle rate. At 200 atmospheres and 925.degree. F. an increase in inerts from 1.5 to 15% decreases the equilibrium ammonia content from 19 to 15%.
Catalyst activity is another variable affecting reactor performance. High activity is desirable in that it allows one to decrease operating temperature and thereby increase the thermodynamic equilibrium ammonia content and the extent of conversion to ammonia. The catalytic material utilized in present day ammonia plants is the long established alkalized magnetite catalyst, and an increase in activity has only been obtained by a decrease in its particle size. Utilization of the higher activity of smaller particle size catalyst has introduced process design problems in that the smaller particle size increases catalyst bed pressure drop, which is an important consideration in the overall plant design in that it adversely affects the horsepower required for recycling unconverted synthesis gas. In early fixed bed designs a particle size of 6-10 mm in diameter was preferred, though in some designs a particle size as large as 14-20 mm in diameter was used. Over the years the designers have been using smaller and smaller size catalyst to take advantage of the increased activity. The literature, however, does not report any designer using catalyst below 1.5-3 mm in diameter.
It is thus apparent to those skilled in the art that the process design of ammonia plants is a complex balance of the many process variables involved--pressure, temperature, space velocity, catalyst activity, catalyst particle size, the amount of inerts in the synthesis gas feed, and the method of removal of the exothermic heat of reaction. The kinetics of the synthesis reaction is dependent on space velocity, catalyst activity, temperature, and the degree to which the conversion approaches the thermodynamic equilibrium ammonia content. The latter is dependent on temperature, pressure, and the amount of inerts in the gas. It is readily apparent that there are many interactions in the process variables involved.
As a result of these multiple design parameters many different commercial unit synthesis reactor designs have resulted. These are extensively illustrated and discussed in "Fertilizer Science and Technology, Volume 2, Ammonia, Part III." In the early design of ammonia synthesis reactors vertical flow of synthesis gas through the catalyst beds was practiced. In more recent reactor designs attempts have been made to take advantage of the higher activity of the smaller particle size catalysts. In order to decrease the higher pressure drop inherent in the use of the smaller particle catalyst, flow through a thinner bed of catalyst by radial flow is practiced. The M. W. Kellogg Company has developed flow through a thinner bed of catalyst in a horizontal reactor design.
An optimized process design for a modern commercial fixed bed unit is illustrated by Quartulli and Wagner of The M. W. Kellogg Company in their publication entitled "Why Horizontal NH.sub.3 Converters?", Hydrocarbon Processing, page 117, December 1978. This publication shows some of the undesirable characteristics of a typical present day synthetic ammonia process. The horizontal reactor with a capacity of 1712 tons/day of ammonia has three beds of catalyst, the outlets from beds 1 and 2 being cooled by direct quench with cold feed gas. The ammonia content builds up from 1.7% at the inlet of bed 1 to 8.2% at the outlet, to 12% at the outlet of bed 2, and to 13.2% at the outlet of bed 3. The net build up is only 10.1% basis feed. This low conversion has a significant affect on other steps in the ammonia plant process scheme. It is apparent from the low conversion to ammonia that considerable recycle of unconverted feed is required. It can be calculated that the recycle rate is 3.6 times the fresh makeup synthesis gas. The high recycle rate has the effect of increasing the size of the reactor, the horsepower of the synthesis gas compressor, and as well the horsepower requirement of the ammonia refrigeration compressor. The recycle gas stream is combined with the makeup synthesis gas, and the total stream which contains only about 10% ammonia is cooled by ammonia refrigeration from about 100.degree. F. to about -10.degree. F. for the recovery of ammonia. The cooling load is a combination of the cooling and condensation of ammonia and the cooling of substantial quantities of unconverted synthesis gas.
It is apparent from the above that there are a number of areas in an ammonia synthesis plant where investment and operating costs could be minimized. The application of this invention will greatly reduce some of these costs.